Brent Warner, Aerospace Engineer at NASA, discusses the benefits of modeling in the design of cooling systems for satellites and spacecraft.
NASA satellites and spacecraft require fairly advanced temperature control which ensures that the electronic components and highly sensitive instruments work properly. Every piece of equipment has a temperature range in which it works properly. The equipment may be damaged if it gets too far outside its operating temperature range. Much of NASA’s equipment (both in space and on the ground) is designed to work at room temperature. However, some equipment, including some of the sensors used to make astronomical observations, must be cooled to cryogenic temperatures to have the necessary sensitivity.
Some of the systems used to cool those sensors are built by NASA Goddard’s Cryogenics Branch. The coldest systems that the group designs combine a bath of liquid helium, at slightly above 1 kelvin (that is, slightly above 1 degree above absolute zero) with an Adiabatic Demagnetization Refrigerator (ADR), which takes the temperature down to less than 0.1 kelvin.
At atmospheric pressure, liquid helium boils at 4.2 kelvin. At lower pressures, liquid helium boils at lower temperatures. The lowering of boiling point with lower pressure is normal for liquids; cooks know that water boils at a significantly lower temperature at high altitudes than it does near sea level. Similarly, a liquid helium bath can be cooled to around 1 kelvin by reducing the pressure above it. In the laboratory, a vacuum pump lowers the pressure. In space, the surrounding vacuum does the job.
The ADR produces its cooling by the action of a changing magnetic field on a block of a paramagnetic (that is, weakly magnetic) substance. The properties of each substance determine its useful temperature range. Unfortunately, no single substance has a range that stretches from room temperature down to the coldest temperatures that the Goddard cryogenics group must achieve. This explains why the group’s flight ADRs are combined with a bath of liquid helium, which connects to the ‘hot’ end of the ADR.
Although the changing magnetic field is a necessary part of the ADR, the field can create problems if it penetrates other parts of the spacecraft, especially the very sensors that the ADR is intended to cool. The sensors are designed to respond to the faintest of incoming signals. Such sensors may therefore respond also to disturbances arising from other equipment in the spacecraft, including the magnetic field of the ADR. Therefore, the cryogenics group designs shielding to ensure that the magnetic field outside the ADR will be below the limits required for proper operation of the rest of the spacecraft.
The critical cooling of instruments
Although much of NASA’s equipment operates at room temperature, some needs to be cooled to cryogenic temperatures. For example, telescopes which study the skies in the infrared. Infrared light has wavelengths longer than visible light. It is often called “heat rays,” because warm objects (including room temperature objects) radiate in the infrared. Obviously, a telescope designed to study infrared light should not be radiating in those wavelengths.
Given the obvious connection between cooling and infrared, it may come as a surprise that the Goddard cryogenics group developed its first satellite ADR with a helium system for an x-ray instrument, the X-Ray Spectrometer (XRS). There are two ways that the cooling helped the XRS achieve its desired sensitivity: by reducing noise in the electronics and by taking advantage of the properties of the materials used to construct the sensor.
Heat is a random motion of atoms, molecules and electrons. These random motions in the wires, and the other components, cause random noise in the electric currents that make up the signal. By cooling the electronics, many of the random motions that cause noise are eliminated. Thus allowing the instrument to accurately record and measure weaker signals.
The XRS uses microelectronic sensors, constructed by the same methods used for integrated circuits. When an x-ray hits a sensor, the energy of the ray is absorbed, warming the sensor slightly. The amount of warming indicates the energy (and hence wavelength) of the x-ray. The materials properties are such that, the colder the sensor, the greater the temperature change for a given energy input. The greater the temperature change, the easier it is to measure.
Modelling of equipment
To allow NASA Goddard’s Cryogenics branch to develop equipment which can operate at room temperature and at extremely low temperatures while minimising magnetic field leakage, the Aerospace Engineers rely extensively on using engineering software to model, refine and identify improvements before beginning the manufacturing process.
The Cryogenics branch recently switched to using 2D and 3D magnetic field solvers from modelling specialist INTEGRATED Engineering Software. This software enables them to model the magnetic field and shielding requirements of any cooling system.
Brent Warner, Aerospace Engineer within Goddard’s Cryogenics group at NASA, designs cooling systems for satellites. Brent commented, “One of the difficulties of magnetic design is that we often need to know the size of the field in areas where it would be difficult to measure directly. Because the basic physics of magnetic fields is well known, we’re confident that we can get accurate values from the software in those areas.”
NASA has been using software from INTEGRATED since 1992, while Goddard’s Cryogenics branch at NASA started to use it in 2001. Previously it used the Poisson group programs. Brent continues, “These programs are free, but not as easy to use. They use a text file for data input, which was state of the art at the time the codes were first written. In contrast, INTEGRATED’s programs have a more contemporary graphical user interface for data input and manipulation.”
Brent continued, “In Poisson, to enter the details of the model, you have to type a text file, which lists all of the characteristics. You list the entire X, Y co-ordinates of all of the pieces that you are studying and include a list of the magnetic properties, such as iron or other magnetic materials that you are using. With Magneto for example, you just draw a picture on the screen, you can still type in all of the coordinates, but the picture is visible.”
Magneto and Amperes calculate not only the magnetic fields from understood structures such as coils or solenoids, but also the magnetic field caused by other materials and unusual structures that are much more difficult to calculate.
It is also possible to calculate the field of a coil using general mathematical software, assuming that the equations are known. However, that software will not calculate the field when magnetic material such as iron, is added to the model. The solution depends on the magnetic properties of the iron (which are nonlinear) and on the shapes of the iron pieces and their positions in the field generated by the coil. This additional complexity is precisely what specialised magnetic software is designed to accommodate.
Bruce Klimpke, Technical Director at INTEGRATED explained, “With Magneto and Amperes, the user can easily fill in the details of the outline of the shape of the shield, and run the program to see what the results are. This allows engineers or designers to efficiently work out a complex calculation, and quickly change the details, such as one material to another material, or change the thickness or the shape.”
The software enables the user to look at magnetic material, such as the iron being used to create a shield, and identify whether it has been saturated. If the field is too high and it can’t provide any more shielding, this would indicate that the user might need to increase the thickness of the shield material. The shield needs to be as thin as possible, because the apparatus that is being designed to go into space needs to be as light as possible.
Boundary Element Analysis
Magneto and Amperes provide multiple options for the display of results. Brent comments; “This is an advantage because you can quickly switch between displays and study various quantities once the program has calculated the answer.”
The previous Poisson tools, could calculate the magnetic field in a given area – but if the user needed to find out the magnetic field outside that area, they had to input the data, set the problem up and run it all over again. This can be a time consuming process. Magneto and Amperes includes the Boundary Element Method as one of the field solvers in the software package. The user only needs to increase the calculation area and the programme will work out the field readings itself.
Brent commented, “To do the sort of job that we are doing, we need advanced and easy to use software. Both the 2D and 3D software that we use with a graphical user interface helps us get our job done much quicker.”
Brent continues, “This image demonstrates a plot of the magnetic field. I have set the lower limit to display the field at a level of 50 Gauss. (You can see that in the View Range input section.) Where the field is below 50 Gauss, the window background shows through. If I were required to keep the field outside of our cooler below 50 Gauss, then setting that display limit would let me quickly see where I have met that requirement and where I have not.”
“This graphic shows the magnetization which the response of the magnetic material to the field created by the coil. It is the response of the material that creates the shielding effect. Every material has a maximum magnetization, so its useful to know if the design approaches that value. The magnetization in the ends of the shield is much lower than in the middle. This suggests that we could save weight and space by using thinner end caps. That’s the sort of design decision you could make by studying a display of the magnetization.”
Brent further explains, “This graphic shows a graph of the B field along the axis, from the center, out to a point 12 cm away from the center. Magneto created this graph when I set it to display the graph along a segment, then clicked on the line that’s in the left side of all the other pictures. In Magneto, if there is a place where you know ahead of time that you will want to see a graph of some quantity, you can draw the line while you’re drawing the model. Then, when you have the model solved, you can simply tell the display section to create a graph along a segment, click on the segment, and you’ve got your graph.”
The systems in the above pictures are not finished designs. They represent the early stages in the design process. Usually, the requirements that go into a design include:
- A limit on the “fringing fields”, which is the magnetic field in areas outside the cooler. The limit is because other equipment is sensitive to the magnetic fields.
- A specification on the field strength inside the coil at the location of the “salt pill”, which is shorthand for the block of paramagnetic substance.
- A weight limit, in the case of coolers that will fly in a satellite. If the original design is too heavy, the designer needs to identify places where the shield can be reduced.
Brent adds, “In designing a shielding system, I’d probably go back and forth with the user of the system 15 or so times, adjusting various dimensions to get things right. All of the design decisions affect other design elements. For example, the shield affects the field in the center of the coil. So, if you were originally planning on running the magnet with a particular current to get a particular field, then when you add the shield, you would need to decrease the current in the coil to drop the field back to the size you want. But, when you drop the current to the magnet, that would decrease the field everywhere, including the fringing fields. So you might not need as thick a shield.”
Bruce Klimpke concludes, “Engineering software is hugely beneficial to designers. It allows engineers such as Brent Warner to model equipment to extremely specific and sensitive specifications, without actually having to physically build anything, thus saving significant time and money. This is particularly important in the current economic climate, where organizations are tightening their belts.”